Hydrolysis and biological degradation of atrazine in soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ns064888j

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  • Detoxification of atrazine in soils results from both chemical hydrolysis and microbial degradation. Infrared analysis was used to study the hydrolysis of atrazine upon interaction with soil colloids and to ascertain the existence of enol, keto, and protonated-keto forms of hydroxyatrazine. Evolution of ¹⁴CO₂ from ¹⁴C-atrazine and ¹⁴C-hydroxyatrazine was indicative of microbial degradation in soils. Objectives of this investigation were: 1) to determine the transitional forms of hydroxyatrazine in different pH environments; 2) to establish the interactions of atrazine on H⁺, Al³⁺, Cu²⁺- or saturated surfaces of "allophane, " montmorillonite, or "natural montmorillonitic clay"; and 3) to ascertain the contribution of microbial degradation and chemical hydrolysis to atrazine detoxification in three Oregon soils. Infrared spectra provide evidence for the existence of enol, keto, and protonated-keto forms of hydroxy-s-triazines. The following transition is correlated with changes in pH: 1) an anionic species at pH > 11.5, 2) an enol form between pH 11.5 and 3.3, 3) a keto form at pH < 3.3, and 4) a protonated-keto species at pH < 0. Asymmetrical side chains (ethyl and isopropyl) of hydroxyatrazine apparently induced a doublet at 3400 and 3520 cm⁻¹ (protonated-ring vNH), whereas the symmetrical side chains (ethyl) of hydroxysimazine yielded a single band at 3330 cm⁻¹ in the protonated-keto forms. Hydroxypropazine was not protonated in these experiments. Acidic cations (H⁺ and Al³⁺) on the exchange complex of montmorillonite and Coker soil clay promoted the hydrolysis of atrazine as evidenced by a strong hydroxyatrazine carbonyl band at 1745 cm⁻¹ in infrared spectra. Reaction of atrazine with Ca- or Cu-montmorillonite did not produce a 1745 cm⁻¹ band, whereas a small degree of hydrolysis of atrazine was indicated in Cu-Coker clay by a weak band at 1745 cm⁻¹. Dehydration increased the hydrolysis of atrazine as evidenced by a more intense band at 1745 cm⁻¹ in the reaction product of Ca- or Cu-Coker soil clay plus atrazine, whereas the infrared spectra of Ca- or Cu-montmorillonite plus atrazine were not affected by dehydration. An "allophanic" colloid did not catalyze the hydrolysis of atrazine when the exchange complex was saturated with H⁺, Al³⁺, Ca²⁺, or Cu²⁺. "Al-allophane" was not sufficiently acidic to protonate added hydroxyatrazine as a carbonyl band was not observed in the reaction product. Thus under acidic field conditions, one might expect the smectites to enhance the chemical hydrolysis of atrazine while "allophanic" colloids and perhaps other amorphous materials would be relatively inert. Respired ¹⁴CO₂ from the ¹⁴C-ethyl side-chain component of atrazine represented approximately 10% of the input ¹⁴C-activity in Parkdale-A and Woodburn soils and 4.5% in Parkdale-C and Coker soils after 28 days of incubation. The isopropyl side-chain and the ring constituent of atrazine were subject to minimal attack by soil microorganisms. The hydroxyatrazine ring was attacked more readily than the atrazine ring. Hydroxyatrazine accounted for approximately 10% of the extracted ¹⁴C-activity from ¹⁴C-atrazine-treated Parkdale-A, Parkdale-C, and Coker soils, and 40% from the Woodburn soil. Hydrolysis is considered the dominant pathway of detoxification in the Woodburn soil, whereas detoxification of atrazine in Parkdale-A, Parkdale-C, and Coker soils is a combination of microbial and chemical activity.
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